1. Technical Field of the Invention
The present invention relates to the recovery of hydrogen (H2) from refinery, petrochemical and chemical gas streams. More particularly, some of these streams are sent to a common fuel gas header. Recovery of the H2 present in these streams produces savings in the operating costs. This invention relates to a method to cryogenically recover hydrogen and hydrogen with liquefied petroleum gas (LPG) from a fuel gas stream.
2. Description of the Prior Art
Hydrogen is an important consumable in hydrocarbon processing to refine oil products and petrochemicals. Hydrogen is also used for refining other chemicals and for food processing. Most hydrogenation and hydrotreating processes require hydrogen at relatively high purity. Some hydrocarbon processes export relatively low purity hydrogen that is usually recovered and recycled for use in processes without high hydrogen purity requirements. The recovery of hydrogen at very high purity is done with the use of adsorption processes, such as pressure swing adsorption, which delivers a hydrogen purity of 99.99% hydrogen. Adsorption technologies are usually associated with relatively large pieces of process equipment, such as pressure vessels, and typically contain proprietary adsorbents, such as zeolites. Both of these characteristics associated with this type of technology result in high capital and operating costs.
In some processes however, it would be more economical to achieve a higher yield of hydrogen at a lower purity. Cryogenically recovering hydrogen from fuel gas streams, as described in greater detail below, achieves a recovery level in the range of about 99.5% with a hydrogen purity of around 95%. Cryogenic hydrogen recovery within an LPG recovery process would be desirable to increase the desirable products to be recovered from a fuel gas stream and reduce operating and capital costs since the process units are combined.
Others have attempted to recover hydrogen from various types of hydrocarbon streams in the past. An example process can be found in U.S. Pat. No. 4,756,730 issued to Stupin. In Stupin, two or more industrial by-product hydrogen gas streams are first segregated by type to produce two feed streams for the process. One of the feed streams combines all of the by-product hydrogen gas streams containing detrimental amounts of non-readily condensable impurities having boiling points below that of methane, e.g., nitrogen, helium, and the like. The other feed stream combines all of the by-product hydrogen gas streams that are substantially free of non-readily condensable impurities. The two feed streams are then separately passed through successive cooling and separation stages. At each separation stage, a liquid bottom fraction containing readily condensable hydrocarbons is separated from the remaining overhead gas of each of the two feed streams. Successive separations are carried out until the overhead streams, which are substantially free of non-readily condensable impurities, achieve the desired degree of purity. When this occurs, the bottom fraction of this stream is primarily liquid methane and is used to scrub a majority of nitrogen and like impurities from the overhead of the streams containing significant amounts of these non-readily condensable impurities. The process in Stupin requires additional process equipment to perform each of the separation steps with recovered hydrogen purity of about 90%. The capital costs associated with installing the needed equipment for this process can be relatively high.
In addition to processes for recovering hydrogen, processes for purifying hydrogen have also been developed. An example process for cryogenically purifying hydrogen is described in U.S. Pat. No. 3,628,340 issued to Meisler et al. In Meisler, the feed gas stream typically contains between 45 and 65 percent hydrogen at a pressure of between 400 and 900 psia. Meisler separates condensable contaminants, such as methane, from a crude hydrogen stream by utilizing a series of multipass heat exchangers through which the gas flows for stepwise cooling, with interstage separation of condensates that are expanded and passed in a reverse flow path for autogenous refrigeration. Supplemental refrigeration can be provided for the last cooling stage to maintain the plant in proper heat balance for variations in feed gas composition and to facilitate startup. Meisler's process is useful for only limited feed gas specifications and requires substantial process equipment to perform the described series of separations and to keep each separate expanded condensate of the respective fractions in its own effluent vapor line. This leads to high capital costs, maintenance issues, and large space requirements.
Others have developed processes for recovering refrigeration, liquefaction, and separation of various products besides hydrogen. An example of such a process can be found in U.S. Pat. Nos. 6,105,390 and 6,425,263 issued to Bingham et al. (collectively “Bingham”). The process of the Bingham Patent is directed to a process for recovering refrigeration, liquefaction, and separation of gases with varying levels of purity. In the Bingham Patent, the feed stream is cooled and then separated into a vapor and a liquid stream. The liquid stream is then sent to an expander where the liquid stream is cooled and sent to the inlet cooler, thereby providing refrigeration to cool the inlet gas. The cycle is then repeated until all of the component gases are separated from the desired gas stream. The final gas stream is then passed through a final heat exchanger and expander. The expander decreases the pressure on the gas stream, thereby cooling the stream and causing a portion of the gas stream to liquefy within a tank. The portion of the gas that does not liquefy is sent back through each of the heat exchangers as a refrigerant. As in the Stupin Patent, the process in Bingham requires additional process equipment to enable the stream to be separated enough times to achieve the desired purity of the stream.
A need exists for a more economical and efficient method of increasing the amount of hydrogen that is recovered from a fuel gas stream. It would be desirable to add the hydrogen recovery process to an existing process, such as a hydrogenation plant that uses hydrogen. A process apparatus to increase the amount of hydrogen recovered from a fuel gas stream without having to add extra equipment, which increases capital and operating costs associated with the process, would be advantageous. Additionally, it would be advantageous to add the hydrogen recovery process to an existing process, such as hydrogenation processes.